Sludge thickener and method

ABSTRACT

The invention relates to a system for the thickening of sludges, in particular domestic sewage or industrial waste sludges. The system comprises two or more consecutively arranged thickening stages each consisting of one thickening tank or several parallel thickening tanks, and wherein the ratio of the largest inside diameter to the mean sludge level is larger than 1:1 in the first tank and smaller than 1:1 in the second tank relative to the direction of sludge flow.

D United States Patent [.151 3,684,092 Busse et al. 5] Aug. 15, 1972[54] SLUDGE THICKENER AND METHOD 2,348,125 5/ 1944 Greem. ..2l0/294 X[72] Inventors: Oswald Busse; Hugo Klesper, both 2'850'449 9/1958Tmpeymm "210/10 Mchelbmh Germany v FOREIGN PATENTS 0R APPLICATIONS [731Asslgnee Mchelbacher 808,906 2/1959 Great Britain ..2l0/l0 221 Filed:Feb. 24, 1970 OTHER PUBUCMTONS N a 13 62 Dorr Thickeners, The DonCompany, Engineers, [211 5 Stamford,,Conn., 1951, pages 2- 5, 16- 21. p

[ Foreign Application Data Primary Examiner-Reuben Friedman March 4,1969 Germany ..P 1911 007.7 Assistant Examiner-Thomas -WYAttorne'y-Larson, Taylor and Hinds [52] US. Cl. ..2l0/73, 210/83,210/201,

, 210/252, 210/513 ABSTRACT [51] Int. Cl. ..B0ld 21/00 The inventionrelates to a Sy for the thickening of [58] held of Search ..2l0/l0, 67,73, 83,96,201, sludges in particular domestic sewage or industrial210/252 2 513 waste sludges. The system comprises two or moreconsecutively arranged thickening stages each consisting [56] Referencescued of one thickening tank or several parallel thickening UNITED A1133PATENTS tanks, and wherein the ratio of the largest inside diameter tothe mean sludge level is larger than 1:1 in 3,036,713 5/1962 Lambeth..2lO/73 X the first tank and Smaller than 1:1 in the Second tankglbemon 2 1138 relative to the direction of sludge flow. I orr v3,259,566 7/1966 Torpey ..2l0/l0 17 Claims, 2 Drawing Figures 8 V l6 24% m Ii 4 9 t::'::' 2 9 ne I5 3 rmcxemwe /2 r TJ l3 O IO? M l4 'HWINHI Qm PATENTEDAHG 1 1912 35841192 SHEET 1 BF 2 INVENTORS OSWALD BUSSE HUGOKLESPER ATTORNEYS PATENTEUAUG 15 m2 sum 2 or 2 mvaumns OSWALD BUSSE HUGOKLESPER BY 2 5 ATTORNEYS SLUDGE THICKENER AND METHOD BACKGROUND OF THEINVENTION This invention relates to sludge thickeners. Sludge thickenersare used to a large extent in municipal as well as industrial wastewater treatment plants for dewatering sludges resulting, for example, insettling tanks and from wet cleaning or mineral raw material. A sludgethickener comprises a basin in which the water content of the sludges isreduced by sedimentation of solids. Stirring mechanisms are frequentlyapplied to facilitate thickening and the removal of gases. After thesludge is thickened, it is subsequently treated to remove more water forexample by natural dehydration on drying beds or by mechanicaldewatering for instance in pressure filters.

The dewatering efficiency of a sludge thickening tank is moreoverlargely dependent on the ratio of the inside width of the tank or incommonly used circular tanks of the tank diameter, to the mean sludgelevel. In past years opinions difiered as to the preference of low orhigh thickening tanks. It was found, however, that thickening efficiencyis primarily dependent on the load and deteriorates with increasingload. Accordingly, wide tanks i.e. those whose diameter by far exceedsthe sludge level were preferred and thus are applied in all cases.

A serious disadvantage of all sludge thickeners, especially those with ahigh dewatering efficiency, is that the subsequent sludge treatmentfacilities (usually pressure filters) require an intermittent,batch-type withdrawal of the sludge from the sludge thickener. Even ifthe influent to the sludge thickener is constant, the sludge levelchanges due to batch withdrawal and the change in level is detrimentalto the thickening effect. Most importantly, in intermittent withdrawal,there is always the risk that the considerably thickened and viscoussludge cannot flow rapidly enough towards the outlet and the supernatantliquor or slurry breaks like a funnel through the sludge into theoutlet. Thus, large sludge volumes with unduly. high water content arewithdrawn and extra load is put on the subsequent treatment plant. Byarranging a funnel-shaped sludge sump at the bottom of a thickeningbasin and by installing a sludge scraping mechanism, this problem canonly partly be resolved. These measures, are generally insufficient andparticularly in thickening tanks with high densifying efficiency.

BRIEF SUMMARY OF THE PRESENT INVENTION In order to resolve the foregoingproblems in a sludge treatment plant of the previously described type,there is provided according to the invention a multi-stage systemwherein the ratio of the largest inside width to the mean sludge levelis greater than 1:1 in a tank of a first stage and smaller than 1:1 in atank of a subsequent stage or at least in the last thickening stagerelative to the direction of sludge flow. Thereby, the advantages of thewide thickening basin with low surface loading are fully exploited inthe first stage, whereas in the second stage the considerably thickenedsludge is treated in tall thickening tanks where it is exposed to highcompression pressure, the sludge column being so high that intermittentwithdrawal will not cause the water to break in. It is particularlyuseful to keep the said ratio between 1:5 to 1:2 in the secondthickening stage according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments of theinvention are included in the following detailed description includingthe drawings in which:

FIGS. 1 and 2 are diagrammatic views of sludge thickening systemsaccording to the invention.

The two-stage thickener plant shown in F IG. 1 comprises twoconsecutively arranged thickener tanks 1 and 2. Thickener tank 1 of thefirst stage is of the conventional, flat design type, its diameteramounts to at least twice the normal sludge level 3. Thus, the load onthe thickener floor is kept low. A scraper bridge 4 traveling on top ofthe cylindrical tank wall carries a stirring device 5 consisting ofrabble arms and a bottom scraper mechanism 6 for conveying the sludgetowards a funnel-shaped sludge sump 7. The height of the sludge sump isabout 1 to 2 meters; The sludge con veyed by conventional conveying;means is fed through inlet conduit 8 into the thickening tank 1 via aconventional inlet distributor 9. The separated water is removed bysuitable overflow means (not shown) and the thickened sludge iswithdrawn by a pump 10 the delivery of which is regulated by a controldevice 11 which is actuated by the torque opposing the stirring device5. The sludge level in tank 1 is thus kept constant. Pump 10 has aconstant delivery independent of the torque-dependent control ensuring asubstantially constant withdrawal of sludge from tank 1.

Thickening aids such as weighting minerals, e.g., ash, or chemicalsinfluencing the sludge consistency such as ferric chloride or aluminumchloride, lime, waste acid or polyelectrolytes, can be added in a mixingtank 12, e. g. a mixing cyclone.

The sludge from tank 1 is then fed into the tank 2 of the second stagewhich is of the high design type, i.e. its diameter is only a fraction,e.g. one-half to one-fifth, of the sludge level. The sludge isintroduced into this tank 2 intermediate the top and bottom thereof atapproxi mately midway therebetween via a centrally arranged distributor13 assuring even distribution of the relatively viscous sludge andeliminating flow irregularities. Tank 2 is also equipped with a bottomscraper blade 14. A stirring mechanism can usually be dispensed withbecause the high sludge column in this tank exerts a heavy compressionpressure. The separated water is discharged via an overflow weirarrangement 14 and returned via pipe 15 into the influent to tank 1. Theamount of sludge introduced into tank 2, and thus the height of thethickened sludge in tank 2, is conveniently controlled by atorque-dependent control 16 which regulates pump 10. Alternatively, thecontrol 16 can comprise a liquid level device to actuate a valveblocking sludge introduced to distributor 13.

The second thickening stage is followed by a pressure filter 17 fordewatering of the sludge; a pump 18 charges the filter in intermittentoperation.

FIG. 2 shows the plant with the thickener tank 21 of the first stage inwhich the diameter-sludge level ratio is also very large. The stirringmechanism, scraping device, and feed well are omitted in this drawingfor simplicity. The partially thickened sludge is withdrawn from thesludge hopper of tank 21 by a pump 20 and conveyed via the mixing tank22 into the second thickening stage. This second stage consists of 3consecutively arranged thickening tanks 23, 24, 25 which are very highrelative to their diameter. These thickening tanks of the second stageare suitably made of plastic tubes in vertical juxtaposition. Theplastic tubes can also be arranged in groups. The tanks of the secondthickening stage are so arranged in consecutive order that the sludgewithdrawn from the funnel-shaped sludge sump of the foremost tank is fedinto the following tank at a point approximately midway between the topand bottom of same. From the last tank 25, the sludge is conveyed intothe pressure filter 27. The overflow means 31, 32, 33 of the secondstage thickener tanks are so arranged that the sludge level graduallydecreases from one tank to the next and the sludge is fed from tank 23into tank 24 and from tank 24 into tank 25 by hydrostatic pressure only.The sludge withdrawn from the overflow is returned via pipe 26 into theinfluent conduit 8 of the first thickening stage.

Significant advantages are achieved by the present invention if aconveyer system is provided ensuring a substantially continuouswithdrawal of the thickened sludge from the first thickening stage. Inthis way, the sludge level, hydrostatic conditions, flow conditions, andsimilar factors influencing the thickening effect of the first stage canbe kept constant regardless of the operation and intermittent chargingof a subsequent sludge treatment plant. In particular, it is possible toavoid sludge withdrawal peaks in the first stage thus eliminatingpenetration of water to which this stage is prone because of therelatively low sludge level. It is useful to provide control deviceswhich regulate the sludge inlet and/or outlet of the first thickeningstage in order to maintain a constant sludge level in the firstthickening stage. These devices which are intended for controlling theactual level of the sludge layer and not primarily that of thesupernatant liquor, are preferably actuated by a measuring device whichmeasures the torque acting upon a stirring and/or scraping mechanismwhich operates within the sludge.

Because of the intermittent discharge and feed-in of sludge into apressure filter or similar system, fluctuations of the sludge level inthe thickening tank of the second stage cannot be avoided. Thesefluctuations, however, are not decisive because of the considerablyhigher absolute sludge level. Also in this case, it will of course beattempted to keep the conditions as constant as possible. It isexpedient to provide control devices particularly in the second stageregulating the sludge influent or water discharge so as to preventexceeding the maximum sludge level. This can either be achieved byinstallation of a level control switch blocking the sludge influent orby providing an overflow arrangement in the second thickening stage toconvey overflow to the influent of the first thickening stage.

The sludge leaving the first thickening stage and entering the second isalready quite dense, therefore it is preferred to feed the sludge into azone of the second thickening stage where approximately the sameconditions prevail. According to a preferred feature of the presentinvention, the sludge efiluent pipe discharges into the secondthickening stage in medium water depth. It is also preferable that thesludge pipe discharges via inlet distributors into the second thickeningstage, thus even distribution is ensured and irregular flow iseliminated.

Excellent results have been obtained where the second stage comprises acircular basin 6 to 9 meters high, preferably 8 meters high, and 3 to 5meters, preferably 4 meters, in diameter. The second or at least thelast thickening stage respectively preferably comprises severalindividual tanks parallel to each other and consisting of verticallydisposed plastic tubes rather than one single basin. in that case thediameter sludge level ratio according to the invention is applicable toeach individual tank. This results in a particularly simple andinexpensive construction of thickening tanks as plastic tubes aremanufactured in mass production.

A method for operating a two-stage or multiple-stage sludge thickeningplant of the said type according to the invention comprises intermittentwithdrawal of sludge from the second or last thickening stagerespectively, thus ensuring the maintenance of a substantiallycontinuous sludge removal from the first thickening stage.

The system according to the invention furthermore offers the possibilityof admixing the sludge with thickening aids, in particular weightingminerals such as ash and/or chemicals which influence the consistency ofthe sludge. Such additives are preferably dosed between the first andthe second thickening stage as they are particularly desirable in thesecond thickening stage.

What is claimed is:

1. Apparatus for thickening sludges comprising: a first stage comprisinga first thickening tank including tank bottom scraper means forconveying sludge towards a bottom outlet, means for maintaining sludgeintroduced into said first tank at a level adjacent the top thereof, theratio of the largest inside dimension of said first tank to its heightup to said sludge level therein being greater than 1:1; and a secondstage comprising a second thickening tank including tank bottom scrapermeans for conveying sludge towards the bottom outlet, means forregulating sludge introduced into said second tank at a level adjacentthe top thereof, the ratio of the largest inside dimension of saidsecond tank to its height up to said sludge level being less than 1:1and sufiiciently low such that the tendency for supernatant liquor abovethe sludge in said second tank to break through into the sludge outletupon withdrawal of sludge from said second tank is substantiallylessened relative to withdrawal of the sludge from said first tank;means for conveying untreated sludge to said first tank, means forconveying thichkened sludge from said first tank to said second tank,and means for intermittently removing thickened sludge from said secondtank.

2. Apparatus according to claim 1 wherein said first and second tanksare cylindrical, and the ratio of the diameter of the first tank to itsheight up to said sludge level is greater than 2: 1, and the ratio ofthe diameter of the second tank to its height up to said sludge level isbetween 1:5 and 1:2.

3. Apparatus according to claim 1 further including means forcontinuously conveying thickened sludge from said first tank to saidsecond tank.

4. Apparatus according to claim 3 wherein said first tank furtherincludes a stirring mechanism and said first sludge tank levelmaintaining means comprises measuring means responsive to torque actingupon said stirring mechanism.

5. Apparatus according to claim 1 wherein said sludge level regulatingmeans comprises means to regulate the amount of sludge conveyed to saidsecond tank.

6. Apparatus according to claim 5 wherein said sludge level regulatingmeans is responsive to a torque dependent switch.

7. Apparatus according to claim 5 wherein said sludge level regulatingmeans is responsive to a level regulating switch.

8. Apparatus according to claim 1 further including means to conveyliquid overflow from said second tank to said first tank.

9. Apparatus according to claim 1 further including 11. Apparatusaccording to claim 1 further including.

means for introducing thickening aids into the sludge conveyed from saidfirst tank.

12. Apparatus according to claim 1 wherein said first thickening tankincludes a funnel-shaped sludge sump.

13. Apparatus according to claim 2 wherein said second tank has adiameter of from 3 to 5 meters and a height of from b 6 to 9 meters.

14. Apparatus according to claim ll wherein said second stage includes aplurality of said second tanks, and conduit means to convey sludgesequentially through said plurality of tanks.

15. Apparatus according to claim 14 wherein said second tanks arefabricated of plastic material.

' 16. A method of thickening a raw sludge comprising the steps ofintroducing a raw sludge in a first cylindrical settling zone,maintaining the depth of said sludge less than the diameter of saidfirst cylindrical settling zone; removing thickened sludge from thebottom of said first zone, conveying said removed thickened sludge to asecond cylindrical settling zone, maintaining the depth of said sludgesufficiently greater than the diameter of said second cylindricalsettling zone such that the tendency for supernatant liquor above thesludge in said second tank to break through into the sludge outlet uponwithdrawal of sludge from said tank is substantially lessened relativeto withdrawal of the sludge from said first tank, and intermittentlyremoving thickened sludge from the bottom of said second zone.

17. A method according to claim 16 wherein the thickened sludge iscontinuously removed from the first zone and intermittently removed fromthe second zone.

2. Apparatus according to claim 1 wherein said first and second tanksare cylindrical, and the ratio of the diameter of the first tank to itsheight up to said sludge level is greater than 2:1, and the ratio of thediameter of the second tank to its height up to said sludge level isbetween 1:5 and 1:2.
 3. Apparatus according to claim 1 further includingmeans for continuously conveying thickened sludge from said first tankto said second tank.
 4. Apparatus according to claim 3 wherein saidfirst tank further includes a stirring mechanism and said first sludgetank level maintaining means comprises measuring means responsive totorque acting upon said stirring mechanism.
 5. Apparatus according toclaim 1 wherein said sludge level regulating means comprises means toregulate the amount of sludge conveyed to said second tank.
 6. Apparatusaccording to claim 5 wherein said sludge level regulating means isresponsive to a torque dependent switch.
 7. Apparatus according to claim5 wherein said sludge level regulating means is responsive to alevel-regulating switch.
 8. Apparatus according to claim 1 furtherincluding means to convey liquid overflow from said second tank to saidfirst tank.
 9. Apparatus according to claim 1 further including sludgeinlet means provided approximately midway between the top and bottom ofsaid second tank to receive thickened sludge conveyed from the firsttank to the second tank.
 10. Apparatus according to claim 7 wherein saidsecond tank includes a sludge distributor located centrally of saidtank.
 11. Apparatus according to claim 1 further including means forintroducing thickening aids into the sludge conveyed from said firsttank.
 12. Apparatus according to claim 1 wherein said first thickeningtank includes a funnel-shaped sludge sump.
 13. Apparatus according toclaim 2 wherein said second tank has a diameter of from 3 to 5 metersand a height of from b 6 to 9 meters.
 14. Apparatus according to claim 1wherein said second stage includes a plurality of said second tanks, andconduit means to convey sludge sequentially through said plurality oftanks.
 15. Apparatus according to claim 14 wherein said second tanks arefabricated of plastic material.
 16. A method of thickening a raw sludgecomprising the steps of introducing a raw sludge in a first cylindricalsettling zone, maintaining the depth of said sludge less than thediameter of said first cylindrical settling zone, removing thickenedsludge from the bottom of said first zone, conveying said removedthickened sludge to a second cylindrical settling zone, maintaining thedepth of said sludge sufficiently greater than the diameter of saidsecond cylindrical settling zone such that the tendency for supernatantliquor above the sludge in said second tank to break through into thesludge outlet upon withdrawal of sludge from said tank is substantiallylessened relative to withdrawal of the sludge from said first tank, andintermittently removing thickened sludge from the bottom of said secondzone.
 17. A method according to claim 16 wherein the thickened sludge iscontinuously removed from the first zone and intermittently removed fromthe second zone.